1. Field of the Invention
The present invention relates to photoneutralized, pH sensitive, aqueous polymeric dispersions.
2. Description of Related Art
Free radical polymerizable compositions offer a number of advantages over heat or ambient temperature curing, including: rapid curing at ambient temperatures; elimination of solvents and the environmental problems associated with there use; elimination of solvent recovery; elimination of the use of fossil fuels for curing and their polluting effects; and the capability of coating heat sensitive substrates. However, radiation curing at times introduces its own difficulties, such as in connection with formulating compositions having varying degrees of viscosity; use of toxic components and inhibition of curing by air contact. These difficulties may be substantially overcome by selective formulation of the radiation curable composition, and hence, the selection of the components of the radiation curable composition becomes critical.
Radiation curable compositions are well known in the art. Monomers typically used for such purpose include acrylic and methacrylic acid esters of various diols and triols, such as 1,6-hexanediol, diethylene glycol, 1,4 butanediol, trimethoxypropane, pentaerythritol or glycerol, along with alkoxylated monomers, such as ethoxylated and propoxylated derivatives thereof. Typical examples of radiation curable compositions employing some of the above-described monomers have been described in U.S. Pat. Nos. 3,594,410 and 3,380,831, which are concerned with printing and thermal transfer reproductive elements. U.S. Pat. Nos. 3,912,670; 4,025,548; 4,183,796; 4,243,500; 4,360,540; and 4,404,075 teach radiation curable coating and adhesive compositions. It is also well known to prepare radiation curable compositions using specialized polymerizable materials. For example, U.S. Pat. Nos. 4,994,346; 5,308,744; 5,300,380; 5,585,222; and 5,498,765 teach film forming photoreactive polymeric compositions for use in photoresist materials. The polymeric material employed in each case, however, contains cationic polymerizable groups. U.S. Pat. Nos. 5,206,116; 5,296,332; and 5,384,229 describe film forming photoreactive polymers that also contain cationic polymerizable systems for use in electrodeposition or solder masking. In U.S. Pat. Nos. 3,356,461 and 3,342,787, esters of styrene maleic anhydride copolymers are used. U.S. Pat. Nos. 3,862,067 and 3,884,856 disclose styrene maleic anhydride copolymers derived from low molecular weight copolymers of styrene and maleic anhydride. U.S. Pat. Nos. 3,825,430 and 4,401,793 teach the use of polymerizable esters prepared by reacting an anhydride-containing polymer with an excess of hydroxyalkylacrylate or methacrylate wherein the esters are free of unreacted anhydride groups. French Patent 2,253,772 describes the use of styrene-maleic anhydride polymers having free hydroxyl groups. U.S. Pat. No. 4,293,636 teaches a photopolymerizable composition containing polyester, half esterified hydroxyalkylacrylate of polybasic acid and vinyl monomer. However, all of these references teach radiation curable compositions containing organic solvents.
Representing a departure from solvent based radiation curable compositions, European Patent 0 257 554 A2 teaches a radiation curable composition employing a stable aqueous dispersion of polymers produced from a monomer containing ethylenic unsaturation and pendant cationic ethylenic functionalities. The polymers cross-link upon exposure to radiation in the presence of a cationic photoinitiator. U.S. Pat. No. 4,745,138 describes a radiation curable composition comprising low molecular weight partial ester of free anhydride-containing copolymers. These polymers, however, are not water soluble or dispersible due to the free anhydride functionalities.
For the most part, the prior art does not teach aqueous polymer dispersions or solutions wherein in the presence of a cationic photoinitiator, the polymer precipitates out of the dispersion or solution to form a film upon exposure to radiation.
The prior art does not teach aqueous polymer dispersions or solutions wherein in the presence of a cationic photoinitiator, the polymer precipitates out of the dispersion or solution to form and undergo cross-linking upon exposure to radiation.
It is a primary object of the present invention to provide an aqueous polymer dispersion or solution wherein in the presence of a cationic photoinitiator, the polymer does not polymerize but precipitates out of solution to form a film.
It is also an object of the present invention to provide an aqueous polymer dispersion or solution wherein in the presence of a cationic photoinitiator, the polymer does not polymerize but precipitates out of solution to form a free radical cross-linkable film when exposed to radiation.
Another object of the present invention is to prepare simple and inexpensive printing inks and coatings using these aqueous polymer dispersions.
Another object of the present invention is to prepare simple and inexpensive energy curable printing inks and coatings using these aqueous polymer dispersions.
These an other objects, which will become apparent from the following description of the present invention.
In one aspect the invention is an energy curable basic aqueous polymer dispersion wherein the polymer is substantially free of cationic polymerizable functionalities and capable of being precipitated from the dispersion in the presence of an acid generating photoinitiator upon exposure to radiation.
In another aspect the invention is a water based energy curable coating and ink composition employing these energy curable basic aqueous polymer dispersions.
The invention extends the use of radiation curing technology to the vast and economical array of water based polymer systems used in water base printing and coating. Water base polymeric systems typically contain acidic or basic functional resins neutralized to a certain pH by a base or acid, respectively, in order to solubilize the resin or resin system in water. As in the prior art, the polymers of the present invention are neutralized and are soluble or dispersible in aqueous medium. However, unlike prior art polymers, the polymers of the present invention are substantially free of cationic polymerizable functionalities and thereby avoid cross-linking via a cationic mechanism when exposed to radiation. The polymer, may however, optionally contain free radical polymerizable functionalities rendering it capable of cross-linking via a free radical mechanism upon exposure to radiation.
Three essential characteristic features are required of the polymers suitable for use in the present invention: a) the polymer must be substantially free of cationic polymerizable functionalities; b) the polymer must be soluble or dispersible in a basic aqueous solution and an acid generating photoinitiator; and (c) the polymer must precipitate out of such solution upon exposure to radiation. Polymers suitable for use in the present invention, for example, may include polyamide resins, acrylic resins, acrylated acrylic resins, amino resins, polyester resins, urethane resins, starch, polysulfonate resins, phenolic resins and melamine resins. In addition to meeting these requirements the polymer may optionally containing free radical polymerizable functionalities, for example, the class of compounds meeting these requirements would include copolymers of the general structure: 
wherein: R1 and R2 are independently selected from the group consisting of hydrogen, C1-C20 alkyl, C6-C10 aryl, C7-C14 alkaryl, C4-C12 cycloalkyl and halogen such as chlorine, fluorine and bromine; and preferably are independently selected from hydrogen, methyl, phenyl, benzyl, or C4-C6 cycloalkyl; R3, R4 and R5 are independently selected from the group consisting of hydrogen and C1-C5 alkyl; and preferably are independently selected from hydrogen and/or methyl; R6 is selected from the group consisting of alkyl, aralkyl, alkyl substituted aralkyl and oxyalkylated derivatives of same containing 2 to 4 carbon atoms in each oxyalkylene group, which group may be 1 to 20 and preferably 1 to 6 repeating units; A is a linear or branched divalent C1-C20 alkylene or oxyalkylated derivative thereof as described in connection with R6; and subscripts x, y, z and t are whole numbers such that the sum of x, y, z and t ranges from 3 to 20, with each being equal to or greater than 1.
In general, the polymerizable compositions of the present invention may be liquids or free flowing solids, depending upon their molecular weight, and are characterized by having a number average molecular weight if between about 1,000 and 20,000, preferably between 2,000 and 4,000, an acid number between 100 to 300 and preferably between 180 and 300, an acrylate equivalent per gram value of at least 0.1, preferably between 10 to 50 molar percent, and a glass transition temperature of at least about 40 degrees C and preferably between 50 and 100 degrees C.
The polymers containing free radical polymerizable functionalities, optionally employed in the present invention, may be prepared, for example, by reacting a styrene/maleic anhydride copolymer, a hydroxyl terminated acrylate and a monofunctional alcohol to form a partial ester. Next, any remaining anhydride functionalities would be opened with a water/ammonia mixture. The solvent would then removed through a solvent exchange process.
A reaction scheme for preparing the free radical polymerizable functionalities employed herein, for example, may constitute adding methyl isobutyl ketone (MIBK) under agitation to a styrene maleic anhydride copolymer having an acid number of 480 and an average molecular weight of 1600. The two materials are then heated to approximately 95-110 degrees C. over 1 to 2 hours under a nitrogen blanket. Next, N,N-dimethylbenzyl amine and a monofunctional alcohol such as n-propanol, ethanol or octadecanol are then added to form a polymeric mixture having an acid number between 200 to 210. The nitrogen blanket is then removed and 4-methoxyphenol and N,N-dimethylbenzylamine is added. Over a period of time, for example 60 to 90 minutes, a hydroxy-functional acrylate such as 4-hydroxybutyl acrylate or 2-hydroxy-ethyl acrylate is added until the acid number of the polymeric mixture is between 130 to 140. The polymeric mixture is then distilled and 4-methoxyphenol is added along with ammonium hydroxide and deionized water. The mixture is then heated, for example to 99 degrees C. The MIBK and water are then removed by distillation. When all of the MIBK has been removed, the water is returned to the mixture as a water/ammonia distillate.
The photoinitiators employed in the present invention, are selected from commercially available acid and base generating photoinitiators. While not wishing to be bound by theory, it is believed that the acidic or basic dispersion, which is photogenerated in the aqueous medium by the action of the radiation and photoinitiator, serves to instantly shift the equilibrium of the polymeric dispersion in the medium and cause the polymer to precipitate out of solution forming a film. Thus, where base generating photoinitiators are employed, acidic polymeric dispersions are preferred. Likewise, where acid generating photoinitiators are employed, basic polymeric dispersions are preferred.
Base generating photoinitiators suitable for use in the present invention include carbamates such as 3xe2x80x2,5xe2x80x2-dimethoxybenzoin carbamate, orthonitrobenzyl carbamate; oximes such as orthophenylacetyl acetophenone oxime and 0,0xe2x80x2-succinyl diacetophenone oxime; and inorganic amines such as bromo (pentamine)cobalt(II) chloride, alkylamine cobalt; and complexes of the general structure Co(NH2Z5) (K)n where Z is a methyl or n-propyl group, K is chloride, bromide or perchlorate ion and n is an integer from 1 to 2.
Acid generating photoinitiators suitable for use in the present invention include diazonium salts; sulfonium salts; iodomium salts; ferocinium salts; tetraphenyl-phosphonium tetrafluoro phosphate; phenanthrolium bis-hexafluorophosphate; diphenylsulfoxonium hexafluorophosphate; triphenylselenonium salts; triphenyltelluronium salts; 2,6-diphenyl-4-p-chloro phenylthiopyryliumtetrafluoroborate; and cyclopentadienyl isopropylbenzene Iron (II).
The preferred diazonium salts are selected from 2,5-diethoxy-4-(p-tolylmercapto)benzene diazonium hexafluorophosphate, 2,4,6-trichlorobenzene diazonium hexafluorophosphate, 2,4,6-tribromobenzene diazonium hexafluorophosphate, p-chloro benzene diazonium hexafluorophosphate.
The preferred sulfonium salts are selected from triarylsulfoniumhexafluorophosphate and triarylsulfonum hexafluoroantimonate.
The preferred iodonium salts are selected from diphenyliodoniumhexafluorophosphate, diphenyliodonium hexafluoroarsenate, diphenyliodoniumtrifluoromethane sulfonate, diaryliodoniumtrifluoromethane sulfonate and UV 9310-C (available from GE Silicones, Waterford, N.Y.).
In addition to the optionally radiation curable, photoneutralizable polymer and photoinitiator, small amounts of free radical photoinitiator and water soluble or dispersible diluents may be added to the aqueous dispersions of the invention for improved film performance properties. These water soluble or dispersible diluents include, but are not limited to, vinyl monomers such as lower alkyl esters of acrylic or methacrylic acid including methyl methacrylate, ethyl acrylate, 2-ethylhexyl acrylate, butyl acrylate and isobutyl methacrylate; vinyl esters such as vinyl acetate and vinyl propionate; vinyl halides; and high solvency monomers such as 2,2-ethoxyethoxyethyl acrylate, tetrahydrofurfuryl acrylate, n-laurylacrylate, 2-phenoxy ethyl acrylate, glycidyl acrylate, glycidylmethacrylate, isodecylacrylate, isoctyl acrylate. Other diluents include vinyl aromatics such as styrene, alphamethyl styrene, vinyl toluene, indene and p-tert butyl styrene, fumaric acid, maleic anhydride and nitrogen containing monomers such as acrylonitrile, acrylamide, methacrylamide, N,N-dimethylacrylamide, N-vinylpyrrolidine and N-vinyl caprolactam. These and other suitable diluents are widely known and some are described in U.S. Pat. No. 4,745,138 which is incorporated herein by reference. Generally the diluents and other additives must not be reactive with the photoneutralization of the polymer dispersion, i.e. will not copolymerize therewith, and shall be water soluble or dispersible. It is important that the additives and diluents do not interfere with the polymerization, photoneutralization or precipitation of the polymeric dispersion.
The amount of polymer having substantially free of cationic polymerizable functionalities employed in the energy curable compositions herein ranges from about 5 wt. % to about 25 wt. %; the amount of polymer having free radical polymerizable functionalities used ranges from about 5 wt. % to about 50 wt. %; the amount of cationic photoinitiator used will typically range from about 0.01 wt. % to about 5 wt. %; and the amount of free radical photoinitiator used will typically range from about 2 wt. % to about 10 wt. %.
The curable compositions of the present invention are cured by means of radiation. As used herein, the term xe2x80x9cradiationxe2x80x9d shall mean electron beam, ultra-violet light, gamma rays, etc., but preferably the term means ultra-violet light or electron beam.
The aqueous polymeric dispersions of the present invention dry instantly. Thus, there is no need for water removal at or during curing. When used as a coating or printing ink, they may be applied by any suitable means, such as by spraying, dipping, flow coating, brushing and the like, followed by or simultaneously with irradiation. Since the energy curable compositions of the invention are aqueous based no volatile organic compounds (VOCs) are present which renders the compositions water washable. Also, the problem of transdermal migration of acrylates, common in many solvent based energy curable cross-linked systems, is also eliminated in these aqueous based dispersions.